# Michelson-Morley experiment

I perfectly understand the concept of the experiment, but a small doubt is haunting me. For example, if I run on different distances at same speed, it does not matter what direction I run, my speed will always be the same, nothwithstanding if I am moving against or with the travel of the earth through space. Evidently, if my speed was measured from outer space it would vary, it is normal galilean relativity.

I understand Michelson-Morley experiment, but they performed this experiment on earth, at a building in Cleveland, they did not measure the rays of light from a view outside earth, so why were they expecting different measures if they knew that this light was supposedly travelling with the earth? I could understand if the test was performed on earth and someone from outer space took measures, but the fact that these measures were taken at earth confuses me; I know they expected different measures, but I dont see how since their reference was on earth!

Probably is a very dumb question, as I am an only an attorney with interest in cosmology.

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ghwellsjr
Gold Member
At the time, virtually all scientists believed that light was a fluctuation in a stationary medium which they called the luminiferous ether and that as the surface of the earth moved through this ether it would create a "wind" which they could measure. That was the purpose of MMX.

Do you understand how MMX would have worked if Galilean Relativity? For example, do you understand why the two roundtrip paths that the light took were at right-angles to each other, instead of some other angle like 180 degrees?

I perfectly understand the concept of the experiment, but a small doubt is haunting me. For example, if I run on different distances at same speed, it does not matter what direction I run, my speed will always be the same, nothwithstanding if I am moving against or with the travel of the earth through space. Evidently, if my speed was measured from outer space it would vary, it is normal galilean relativity.

I understand Michelson-Morley experiment, but they performed this experiment on earth, at a building in Cleveland, they did not measure the rays of light from a view outside earth, so why were they expecting different measures if they knew that this light was supposedly travelling with the earth? [..]
Your question makes me wonder what it is that you "perfectly understand" ... perhaps you understand the concept of the method, but certainly not the concept behind it!

Light was assumed to be a wave in a stationary medium through which the earth travels without friction. That is different from your example: if you walk on Earth in December or in June, your speed will be the same with respect to the road. In contrast, light was modelled as a wave, with a speed that is independent of the motion of the source; instead, the speed of light was assumed to be constant with respect to the ether. Consequently if by great coincidence the Earth would at some time be at rest in the ether, half a year later the Earth would be moving with a considerable speed due to its orbit around the Sun.

You can directly read the purpose of those experiments here:

http://en.wikisource.org/wiki/The_Relative_Motion_of_the_Earth_and_the_Luminiferous_Ether

In that first paper Michelson makes a calculation error, but that doesn't matter for the concept.

PS. welcome to Physicsforums

Bill_K
AlfreCabo's question is perfectly valid. The idea that the velocity of light was constant relative to a luminiferous ether was not the only one proposed, and the Michelson-Morley experiment was not the end of the story.

An alternative suggestion was that the velocity of light was in fact constant relative to the source, and as AlfreCabo points out, the Michelson-Morley experiment does not rule this out, since source and detector were both at rest relative to the Earth.

A much later experiment (1964) to test this utilized a high energy beam of neutral pions which decayed into pairs of gamma rays. The speed of the gamma rays emitted forward was measured directly and found to agree with the speed of light emitted from stationary sources.

ghwellsjr
Gold Member
An alternative suggestion was that the velocity of light was in fact constant relative to the source, and as AlfreCabo points out, the Michelson-Morley experiment does not rule this out, since source and detector were both at rest relative to the Earth.
Don't you mean the source and detector were both at rest relative to each other? The Earth has nothing to do with the null result of the experiment if the speed of light were constant relative to the source which is at rest relative to the detector (since they are colocated).
A much later experiment (1964) to test this utilized a high energy beam of neutral pions which decayed into pairs of gamma rays. The speed of the gamma rays emitted forward was measured directly and found to agree with the speed of light emitted from stationary sources.
You make it sound like the one-way speed of the gamma rays was measured without regard to any previous synchronization of clocks. How does that work? Same thing for the speed of light. Do you have a reference for this experiment?

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Bill_K
There's a general discussion of this idea on the Wikipedia page "Emission theory". The 1964 experiment is referred to there as "Alväger et al", and it's cited in a number of other places, but as far as I know the paper itself is not online.

[..] An alternative suggestion was that the velocity of light was in fact constant relative to the source, and as AlfreCabo points out, the Michelson-Morley experiment does not rule this out, since source and detector were both at rest relative to the Earth. [..]
Michelson's first paper on this topic (linked in post #3) clearly shows that those experiments had as purpose to measure the velocity relative to the ether; they were not meant to test for ballistic emission. That was not the experimental concept, as both ghwellsjr and I pointed out.

However, together with Morley he did test for the effect of moving media on the speed of light, here:

http://en.wikisource.org/wiki/Influence_of_Motion_of_the_Medium_on_the_Velocity_of_Light

Although he did not mention it, those results in agreement with Fizeau are incompatible with plausible ballistic emission theories.

And in the first paragraph of his famous paper together with Morley he does mention emission theory but rejects it on grounds of other experiments:

http://en.wikisource.org/wiki/On_the_Relative_Motion_of_the_Earth_and_the_Luminiferous_Ether

Bill_K
We're in complete agreement that although Michelson argued against it on other grounds, the Michelson-Morley experiment was not designed to test the emission theory, and was not capable of doing so.

The Fizeau experiment, as you mentioned, seemed to indicate it was false. However the velocity of water used in the Fizeau experiment was only 7 meters per second, whereas the Alväger experiment tested emission from pions which at 6 GeV were themselves moving at nearly the speed of light.

The Fizeau experiment, as you mentioned, seemed to indicate it was false. However the velocity of water used in the Fizeau experiment was only 7 meters per second, whereas the Alväger experiment tested emission from pions which at 6 GeV were themselves moving at nearly the speed of light.
Earlier you post.
A much later experiment (1964) to test this utilized a high energy beam of neutral pions which decayed into pairs of gamma rays. The speed of the gamma rays emitted forward was measured directly and found to agree with the speed of light emitted from stationary sources.
So the speed of the gamma rays emitted foreward was lightspeed where as emissions from pions at 6 GeV were only moving at nearly the speed of light.
Have I got this right or missunderstood.

Bill_K